Add documentation and a changelog entry

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/ChangeLog.d/add-subjectAltName-certs.txt b/ChangeLog.d/add-subjectAltName-certs.txt
new file mode 100644
index 0000000..487e5c6
--- /dev/null
+++ b/ChangeLog.d/add-subjectAltName-certs.txt
@@ -0,0 +1,6 @@
+Features
+   * It is now possible to generate certificates with SubjectAltNames.
+     Currently supported subtypes: DnsName, UniformResourceIdentifier,
+     IP address, OtherName, and DirectoryName, as defined in RFC 5280.
+     See mbedtls_x509write_crt_set_subject_alternative_name for
+     more information.